1. Field of the Invention
The present invention concerns a circuit by which in a transmitter, consisting of a controlled amplifier of one or several stages, the lowest power levels can be formed and used with good efficiency.
2. Discussion of Related Art
In a transmitter of a radio telephone, class-C amplifiers are generally used, which are characterized by good efficiency, 60 to 80 percent, at high power levels. Because of the great non-linearity of the class-C-type amplifier, difficulties arise in realizing control of the amplifier at low power levels, and in addition, when lower power levels are used, the efficiency of the amplifier is reduced. This causes no harm when a system (or a locality of use) is in question in which the smallest transmission power levels need not be used. The situation will be different, for instance in the digital GMS system covering in future all Europe, in which very low power levels will be in use. If a telephone is going to be used in an airplane, it is necessary to use a low power level in order to prevent potential interference with the aviational electronics. Consequently, probably the lowest power levels, on the order of -17 dBm order of magnitude, will have to be used.
A typical prior art principle block diagram of a transmitter for a GSM radio telephone is presented in FIG. 1. The block diagram only shows the blocks required for understanding the operation. A signal to be transmitted enters the input RFin of a three-stage class-C-type power amplifier. The gain of the amplifier 1 is controlled by a reference amplifier 3, the output of which is filtered before being input in the power amplifier 1. The input signals of the reference amplifier 3 are the voltage derived from a power detector 2, said voltage being proportional to the output voltage RFout of the power amplifier 1, and the control voltage TXC1 derived from the logic section of the telephone. Said blocks 1, 2 and 3 constitute a control loop, which tends to be controlled in a state such that the voltage to be derived from the power detector 2 and the control voltage TXC1 derived from the logic sections of the radio telephone are of equal magnitude.
With the circuitry of the prior art, power levels are obtainable which extend from the maximum level down to about 25 dB below the maximum power level. If a wider power control range is wished, the following difficulties arise: the narrowness and non linearity of the dynamic range of the power detector and the poor controllability of a power amplifier of the class-C-type make the control of low power levels difficult. On small power levels, the efficiency of the amplifier is extremely poor. According to the specifications of the GSM system, the power increase will on low power levels conform to the COS.sup.2 graph and this is difficult with a class-C amplifier.